1. Field of the Invention
The invention relates to a method for correcting lateral force measuring values for determining the cone effect of a vehicle tire.
2. The Prior Art
After production, vehicle tires are subjected to a multitude of tests in order to ensure that minimum tolerances are met, and furthermore, in the case of those vehicle tires which do meet these minimum tolerances, to enable allocation to quality classes. One of these tests consists of determining a cone effect. A cone effect occurs in the case of conically produced vehicle tires or in the case of uneven spring constants. It causes lateral force to be exerted between the rolling vehicle tire and the roadway surface.
In vehicle tire test equipment, the roadway surface is simulated by a load wheel against which surface area a vehicle tire which has been subjected to extension is pressed. The lateral force can be determined by way of measuring sensors which are arranged at the clamping locations of the axle of the load wheel. The cone effect is determined by taking an average value of a lateral force for right-hand rotation and an average value of a lateral force for left-hand rotation of the vehicle tire and dividing the sum of the two lateral forces by two. Normally, these measurements are carried out in one of the two possible alignments in which the marking that comprises the characteristic data of the vehicle tires (called DOT) points upward or downward.
It has been shown that after turning the vehicle tire, i.e. with the marking pointing downward when before it was pointing upward, or vice versa, and after the measurement is repeated, a different cone effect is determined. The same value but with changed preceding signs should be obtained. Furthermore, it has been shown that different values of the cone effect also occur if the vehicle tire establishes contact, laterally offset, at another position of the surface area of the load wheel. The deviations which have been determined are generated by the vehicle tire test equipment itself. They necessitate a correction in order to reproduce the measuring values and allow a comparison of different tires, and of tires tested on different test equipment.
It is an object of the invention to provide a method for correcting lateral force measuring values, and enable reliable correction irrespective of the location where a vehicle tire contacts the load wheel.
The invention is based on the recognition that incorrect measuring values of the lateral forces picked up by the measuring sensors are triggered by radial forces which arise at the measuring sensor due to a non-ideal separation into force components. In addition, these radial forces can comprise differences between the upper and the lower measuring sensor. This phenomenon is also known as cross-talk.
This influence can be determined by determining in advance the different influences on the measuring sensors of the lower and the upper clamping of the rotary axle of the load wheel. A moment of tilt is exerted on the axle of the load wheel in the direction of the operational radial load in two opposite directions. During this time, the cross-talk of the measuring sensors for lateral forces is determined. The moment of tilt is exerted by applying an eccentric axial force on the load wheel, e.g. by applying a weight, and by subsequently carrying out two readings.
In the first reading, the force acts near the tangent with the vehicle tire, i.e. on the front as viewed by the operator. In the second reading, the force acts on the opposite side, i.e. on the rear as viewed by the operator. Subsequently, correction values are determined from the difference between the two readings. In subsequent readings, these correction values are used to correct the lateral forces measured. The corrected lateral force measuring values can then be used to determine the results of the cone effect which are reproducible and comparable, with the influences of the tire test equipment having been eliminated from these results.
Several steps are carried out during complete correction. In a first step, a cross-talk quantity of radial forces acting on the measuring sensors for lateral forces is determined according to the equation:       Rad    1    xe2x80x2    =            (                                                  Lat                              1                ,                front                                      -                          Lat                              2                ,                rear                                                                        Rad                              1                ,                front                                      -                          Rad                              1                ,                rear                                                    -                                            Lat                              2                ,                front                                      -                          Lat                              2                ,                rear                                                                        Rad                              2                ,                front                                      -                          Rad                              2                ,                rear                                                        )        *          S      1      
wherein
Rad1xe2x80x2 denotes the upper cross talk quantity;
Lat1,front denotes the upper lateral force when the force is introduced from the front;
Lat1,rear denotes the upper lateral force when the force is introduced from the rear;
Lat2,front denotes the lower lateral force when the force is introduced from the front;
Lat2,rear denotes the lower lateral force when the force is introduced from the rear;
Rad1,front denotes the upper radial force when the force is introduced from the front;
Rad1,rear denotes the upper radial force when the force is introduced from the rear;
Rad2,front denotes the lower radial force when the force is introduced from the front;
Rad2,rear denotes the lower radial force when the force is introduced from the rear; and
S1 denotes a scale factor.
In a second step, a cone error of a vehicle tire at a specified load is determined according to the equation:   K  =                              CONY          DOT                +                  CONY          NDOT                                      LOAD          DOT                +                  LOAD          NDOT                      *          S      2      
wherein
K denotes an equipment-dependent cone error;
CONYDOT denotes a cone effect when DOT is at the top;
CONYNDOT denotes a cone effect when DOT is at the bottom;
LOADDOT denotes a radial load through the vehicle tire when DOT is at the top;
LOADNDOT denotes a radial load through the vehicle tire when DOT is at the bottom; and
S2 denotes a scale factor.
In a third step, complete correction values Rad1=Rad1xe2x80x2+K/2 and Rad2=xe2x88x92Rad1xe2x80x2+K/2 are determined from the cross-talk quantity Rad1xe2x80x2 and the cone error K. These correction values are used to correct the lateral force measuring values of the upper and lower measuring sensor. This is carried out according to the general equation:
Lat=Latxe2x80x21+Latxe2x80x22
with Latxe2x80x2x=Latx+Radxxe2x80x2*Radx, wherein
Latxe2x80x2x denotes the corrected measuring value;
Latx denotes the measuring value of the lateral force sensor;
Radxxe2x80x2 denotes the cross-talk quantity, radial to lateral;
Radx denotes the measuring value of the radial force sensor;
Lat: denotes the resulting lateral force.
The final equation is as follows:
Lat=Lat1xe2x88x92Rad1xe2x80x2*Rad1/S1+Lat2xe2x88x92Rad2xe2x80x2*Rad2/S1
Lat=Lat1+Lat2xe2x88x92(Rad1xe2x80x2*Rad1+Rad2xe2x80x2*Rad2)/S1
The result of the first step provides a measure for the difference between the upper and the lower measuring sensor. This value does not depend on radial forces acting on the load wheel and can thus be taken over into the memory of a connected computer and can be used for further readings during vehicle tire tests. In the second step, from the cone effect and the load of the vehicle tire pressing against the load wheel, the equipment-dependent cone error is determined from the cone effects at the marking top and bottom in relation to the load. The sum of the cross talk and of the cone error, as determined in the third step, provides a correction value by which the measured lateral force has to be corrected in order to eliminate the equipment-dependent cone error.
If these corrected values are now used for the lateral force instead of the measured values, then the values of the cone effect are identical, irrespective of whether the vehicle tire was measured with the marking at the top or at the bottom and irrespective of the location at which the tire rolls on the surface area of the load wheel.
According to an improvement, the cross-talk quantity of radial forces acting on the measuring sensor for lateral forces is determined at specified intervals and/or is determined anew after maintenance and retooling work. It is possible that as time goes by or during maintenance and retooling work, clamping of the axle of the load wheel or the alignment of the measuring sensors changes slightly, resulting in a change of cross-talk behavior. By way of renewed determination, such changes can be eliminated and it can be ensured that the test equipment always returns reproducible results. Furthermore, the measuring values of the measuring sensors for the radial and lateral forces on the top and bottom clamping of the rotary axle of the load wheel are recorded and processed separately, and summation of the lateral forces is undertaken only after correction of the lateral forces. Based on such separate processing, the influences of cross-talk on the individual measuring sensors can be selected and the required correction values can be obtained.